Method for manufacturing fuse links



June 4, 1963 H. H. HALLAS 3,092,715

METHOD FOR MANUFACTURING FUSE LINKS Filed May 6, 1960 4 Sheets-Sheet 1we 1?:1 A 1 90 6) c5 2 (5 O O D O O O l O O O D O O O 3 O O O D O O O DO O O J O 8 INVENTOR.

Hurry H. HGHCIS H. H. HALLAS METHOD FOR MANUFACTURING FUSE LINKS June 4,1963 4 Sheets-Sheet 2 Filed May 6, 1960 INVENIOR.

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JNI'ENTOR. Hurry H. Hullus June 4, 1963 H. H. HALLAS 3,092,715

METHOD FOR MANUFACTURING FUSE LINKS Filed May 6, 1960 4 Sheets-Sheet 4INVENTOR. Ho rry H. HoHus WWW g United States Patent Ofi 3,092,715METHOD FOR MANUFACTURING FUSE LINKS Harry H. Hallas, Newton, Mass.,assignor to The Chase- Shawmut Company, Newburyport, Mass. Filed May 6,1960, Ser. No. 27,430 13 Claims. (Cl. 2I9117) This invention hasreference to time-lag fuses, and more particularly to time-lag fuseswherein time-lag is achieved by providing an alloy-forming overlay of ametal having a relatively low fusing point, cg. tin, on a base metalhaving a relatively high fusing point and relatively high conductivity,e.g. silver or copper.

It is one object of this invention to provide improved time-lag fuses ofthe above description.

Such fuses are predicated upon metal diffusion governed by certaindifferential equations.

The application of metal diffusion for the purpose of achieving time-lagin electric fuses has been described in a paper by A. W. Metcalf A NewFuse Phenomenon, BEAMA Journal (British) Pt. 1, April 1939, pp. 109-112; Pt. 2, May 1939, pp. 151452. e time-current characteristics of thistype of fuse shown in the above paper, and those of like time-lag fusesbased on work done along the same lines as Metcalf, subsequent toMetcalf, evidence a relatively Wide spread from average values. In otherwords, the time-current characteristic of this type of fuse is not aline but a relatively wide band. While the blowing-currents andblowing-times of any particular fuse design are defined by and liewithin that hand, the exact time actually required by a given specimento blow when carrying a given current cannot be predicted.

It is, therefore, one object of this invention to provide means fornarrowing the band width of the time-current characteristic or, in otherwords, to provide means which are conducive to a higher degree ofuniformity of performance than in prior art time-lag fuses predicated onthe formation of alloys between metals having dissimilar fusing points.

In prior art fuses of the above description, the time-currentcharacteristic is not only a relatively Wide band, but a band whichtends to widen during the useful life of the fuse. This phenomenon isfrequently referred to as ageing.

It is, therefore, another object of the invention to provide means whichresult in both narrowing of the band width of the time-currentcharacteristic and precluding spreading of its width during the lives ofthe fuses.

The total time involved in interrupting a circuit by a time-lag fusewhich is based on metal diffusion comprises three distinct periods,namely l) the period required for fusion of the low fusing point metal,(2) the period from fusion of the low fusing point metal to kindling ofan arc, and (3) the period from kindling of an arc to achieving areextinction.

The first mentioned and second mentioned periods of time vary dependingupon alloy-formation which has taken place before occurrence of theexcess current which causes the fuse to blow, i.e. during themanufacturing process of the fuse, and during the useful life of thefuse.

It is a further object of this invention to provide processes formanufacturing fuse links making it possible to minimize alloy-formationduring manufacturing, and making it possible to rigidly standardizewhatever minimal alloy-formation takes place during manufacturing.

The larger the mass of low fusing point metal, the longer the timesinvolved for a given current to heat that mass to its fusing point. Itfollows therefrom that fuse links required to have substantial lagtimes, call for relatively large masses of overlay metal.

3,092,715 Patented June 4, 1963 ice It is, therefore, another object ofthis invention to provide processes for manufacturing fuse links makingit possible to bond relatively large masses of low fusing point metalsto high fusing point metals, which processes also minimizealloy-formation incident to the bonding operation, and which processesmake it also possible to rigidly standardize and control minimizedalloy-formation.

It is well known from the theory of electric resistance alloys that in aperfectly periodic lattice a beam of electrons moving in a givendirection continues to move in that direction indefinitely. In otherwords, a perfect lattice has no resistance whatever. If the lattice isnot perfectly periodic electrons moving through the lattice structurewill be scattered, from which scattering resistance results. Theperiodicity of a lattice relative increase in resistance may be verylarge.

As a general rule, when considering the manufacture and operation offuse links with overlays of a metal having a relatively low fusing pointplaced on a base metal having a relatively high fusing point,consideration is given but to the diffusion of the high fusing pointmetal into the low fusing point metal. However, the diffusion of the lowfusing point metal into the high fusing point metal is also important inregard to operation as well as in regard to manufacture of the fuse.

Overlays of a relatively low fusing point metal may be low fusing pointoverlay metal into the high fusing point base metal.

Further objects and advantages of this invention will become apparent asthe following description proceeds, and the features of novelty whichcharacterize this invention will be pointed out with particularity inthe claims annexed to, and forming part of, this specification.

For a better understanding of the invention reference may be had to theaccompanying drawings illustrating the invention wherein:

FIG. 1 shows, diagrammatically, an arrangement of parts for carryingthis invention into effect and a fuse link in an initial stage of themanufacturing process thereof;

FIG. 2 shows substantially the same parts as FIG. 1 in another positionthereof, illustrating the final stage of the process indicated in FIG.1;

FIG. 3 shows, in side elevation, another arrangement of parts forcarrying the invention into effect;

FIG. 4 is a front view of the arrangement of parts shown in FIG. 3;

FIG. 5 is a cross-section, on a larger scale, of the productmanufactured by means of the arrangement shown in FIGS. 3 and 4;

FIG. 6 is a top-plan view of a fuse link material manufactured by meansof the arrangement shown in FIGS. 3 and 4;

FIG. 7 is a longitudinal section of a time-lag fuse comprising a fuselink embodying the present invention; FIG. 7a shows a modification of adetail of FIG. 7;

FIG. 8 is a photomicrograph at a magnification of 50x of a cross-sectionof a prior art fuse link of copper having an overlay of tin;

FIG. 9 is a photomicrograph at a magnification of 500x of the samecross-section as shown in FIG. 8;

FIG. 10 is a photomicrograph at a magnification of 50X of across-section of a fuse of copper having an overlay of tin manufacturedaccording to the present invention; and

FIG. 11 is a photomicrograph at a magnification of 500x of the samecross-section as shown in FIG. 10.

Referring now to the drawings, and more particularly to FIGS. 1 and 2thereof, numeral 1 has been applied to generally indicate a source ofelectric current connected to an electric timer by the intermediary ofelectric switch 2. Timer 3 controls the primary circuit 4 of atransformer 5 whose secondary circuit 6 comprises electrodes 7 and 8.Electrodes 7 and 8 are provided with inserts 9 and 10, preferably madeof carbon steel. Insert 10 defines a cavity 10a into which apre-measurcd pellet 11 has been placed. Pellet 11 consists of a metal inthe nature of soft, low fusing point solder, i.e. a metal having arelatively low melting point and a relatively low conductivity. Pellet11 may consist of tin, tinlead alloys, indium if the base metal of thelink is silver, etc. Numeral 12 indicates a relatively thin sheet of ametal having a relatively high fusing point and a relatively highconductivity, e.g. silver or copper.

The process according to this invention is initiated by adjusting timer3 for a given closing time and lowering electrode 7 and insert 9 so asto engage sheet 12. At this time switch 2 is being closed and parts 7, 9are further lowered exerting pressure upon pellet 11. When insert 9engages sheet 12, a current path is established through sheet 12 andpellet 11. The current then flowing through the secondary circuit 6 oftransformer 5 is relatively small on account of the relatively highohmic resistance of pellet 11. The compression of pellet 11 thenprogressively taking place between insert 10 and sheet 12 is largely dueto so-called cold flow, which is more or less enhanced by virtue of thefact that pellet 11 is being heated by the passage of an electriccurrent through it. The moment sheet 12 engages the upper surface 10b ofinsert 10 a current path is established through sheet 12 immediatelyadjacent to the surface 13 between pellet 11 and sheet 12, shunting theflow of current through pellet 11 and having a considerably smallerelectric resistance than pellet 11. As a result, the magnitude of thecurrent in circuit 6 increases and this causes a relatively intenseheating of interface 13, while all other parts of the arrangement remainrelatively cool. Pellet 11 fuses at the interface 13 on account of theheating of interface 13. Since the heat generated is localized at theinterface 13 and can precisely be controlled by timer 3, fusion ofpellet 11 can be limited to an extremely thin layer of pellet 11. Thislayer establishes a fusion bond with sheet 12 without causing anysignificant diffusion of the base metal of which sheet 12 is made intothe material of which pellet 11 is made. The temperature at theinterface 13 can be rigidly controlled to stay below temperatures atwhich a significant diffusion of the low fusing point metal of whichpellet 11 is made into the high fusing point metal of which sheet 12 ismade will occur.

FIG. 2 shows all the parts illustrated in FIG. 1 after compression ofpellet 11 into a semi-spherical overlay on sheet 12. The circuitry shownin FIG. 1 has been omitted in FIG. 2. FIG. 2 illustrates,diagrammatically, the resistance R inside of the gap formed betweenelectrodes 7 and 8. It will be understood that the curve representingthe resistance changes throughout the entire bonding process, and thatthe curve of FIG. 2 refers to an arbitrary selected point of time. Thecontact resistance r, between insert 9 and sheet 12 is lower than thecontact resistance r between insert 10 and pellet 11. The contactresistance r is maximum at the initial stage of the process illustratedin FIG. 1, and decreases propressively as pellet 11 is being compressedand the area of engagement between pellet 11 and insert 10 increased.The magnitude of r depends upon the metals of which parts 11 and 12 aremade, the geometry of pellet 11 and of cavity 10a, surface conditions ofparts 11 and 12, the size and contour of face 10b, and the force bywhich electrodes 7 and 8 are acted upon. The time of current flow andthe intensity of the current are adjusted in such manner that thesurface of pellet 11 remote from sheet 12 and the surface of sheet 12engaged by insert 9 never reach the fusing point of the respectivemetal. It has not been found necessary to water-cool electrodes 7 and 8in order to achieve this end; but under certain circumstanceswater-cooling of electrodes 7 and 8 may be desirable, or necessary.Reference characters r and r have been applied to indicate the totalresistance of pellet 11, and base of metal 12, respectively. It isapparent that the former is considerably higher than the latter. Theinterface 13 is the point of highest resistance, and greatest heatgeneration. It is the only point where fusion occurs.

The area of engagement between insert 9 and sheet 12 is much larger thanthe area of engagement between sheet 12 and pellet 11 and the area ofengagement between sheet 12 and insert 10. Therefore the current densityat the upper surface of sheet 12 will be considerably less than thecurrent density at the lower surface of sheet 12. Hence heating willprimarily occur at the lower surface of sheet 12 and bring the lowersurface of sheet 12 to the fusing point of the metal of which pellet 11is made without need of heating portions of sheet 12 remote from pellet11 to a temperature above the fusing temperature of the metal of whichpellet 11 is made.

Referring now to FIGS. 3 and 4, numerals 14 and 15 have been applied toindicate a pair of rolls supported by shafts 16 and 17, respectively.Rolls 14, 15 are electrodes and are connected to the same circuitry asshown at the left of FIG. 1, i.e. they form part of a secondary circuitof a transformer. Rolls 14, 15 are preferably made of carbon steel. Theupper roll 14 is provided with a peripheral groove 14a which isrectangular in cross-section and adapted to receive a relatively thickand narrow wire or strip 18 of a metal in the nature of soft, low fusingpoint solder, e.g. tin, or an alloy of tin. Strip 18 is taken from arotatable supply reel (not shown). A relatively thin and relatively widesheet 19 of a metal having a relatively high conductivity and arelatively high fusing point, e.g. copper, is fed to the nip N formedbetween rolls 14 and 15. Sheet 19 is moved from right to left, asindicated by the arrow S. This may be achieved by suitable transportrolls not shown in the drawing. Strip 18 is inserted into, and guided bycircular groove 14a in poll 14. Rolls 14 and 15 may either be powerdriven or rotated as indicated by arrows T and T by frictionallyengaging strip 18 and sheet 19. An electric current is caused to flowfrom roll 14 through strip 13, sheet 19 and roll 15. The portion of thatcurrent flowing through strip 18 is relatively small since the specificresistance of the metal of which strip 18 is made is relatively high andsince strip 18 is relatively thick. The current flowing through strip 18is not sufficient to cause fusion of strip 18 at any point thereof. Thepreponderant portion of the current flowing from roll 14 to roll 15fiows through the two flanges 14!), 14b of roll 14 laterally boundinggroove 14a and strip 18. The current density will be highest at thepoints where sheet 19 is engaged by flanges 14b, 14b of rolls 14 andlowest where the lower surface of sheet 19 is engaged by the relativelywide surface of roll 15. Hence heat generation will be concentrated onthe upper surface of sheet 19 and will be limited to two marginal zonesimmediately adjacent the interface 20 formed between strip 18 and sheet19.

By proper selection of all the parameters affecting the process, and inparticular the intensity of the current flowrence of large short-circuit15 and the duration each increment of strip 18 and sheet 19 is heated bythat current, depending in turn on the velocity at which materials 18,19 are moved in the direction of arrow S, fusion of strip 18 can beLimited to a narrow zone immediately adjacent to sheet 19. This has beenindicated in FIG. wherein reference character 18a has been applied tothat portion of the prefabricated strip 18 whose microstructure hasremained unchanged during the bonding process and wherein referencecharacter 18b has been applied to indicate a narrow zone immediatelyadjacent interface 20 that was fused during the bonding process andwhose original microstructure changed thereby.

Referring now to FIG. 6, the sheet which may be a thin and wide copperwith a plurality of spaced parallel lines 19a each formed by a pluralityof contiguous aligned perforations. In FIG. 6 these perforations areshown to. be circular, but they may have any other desiredshapeconsistent with their purpose of defining points of restrictedcross-sectional area where fusion will be initiated at the occurrence offault currents. Strip 18 of a metal in the nature of soft, low fusingpoint solder is arranged parallel to lines 19a formed by perforations,and it adheres to sheet 19 in such a way as to cover a predeterminedportion of the constituent perforation of one of lines 19a. As shown inFIG. 6, strip 18 covers 50% of the area of the constituent perforationsof one line 19a and its right edge 18ccoincides with the point ofminimum cross-sectional area delined by the central line of perforations19a. As a result, the area of the sheet material 19 situated to the leftof edge 18c will be cooled by overlay 18 when sheet 19 is used to carrycurrent whose direction of flow is substantially at right angles toarrow S. The semi-finished product shown in FIG. 6 may be cut intostrips of any desired width, depending upon desired current-carryingcapacity, at right angles to arrow S, and each such strip is adapted tobe used as a fuse link, or fusible elemenhin a time lag fuse. Theparticular arrangement of strip 18 shown in. FIG. 6 makes it possible tominimize the number of break-fonning points for a given voltage andcurrent rating. This is due to the fact that breaks will form at thepoints orline of minimum cross-sections associated with strips 18, bothon occurrence of small protracted overload currents, and on occurence oflarge short-circuit currents. On occurcurrents' the temperature rise atall points of'restricted cross-sectional area, including thoseassociated with overlay 18, will be rapid and the cooling effect ofstrip 18 will be negligible under such conditions. Therefore the fivelines of perforations provided in the structure of FIG. 6 will result inthe formation of five series breaks on occurrence of fault currentsinvolving a relatively rapid rate of rise of current flow. On occurrenceof relatively small protracted overloads strip 18 will operate as aneffective cooling and heat absorbing means for the points of narrowestcross-section of sheet 19 associated with strip 18.

ing between rolls 14,

19 shown therein sheet is provided associated with strip 18 reach andexceed the fusing tem perature of the metal of which strip 18 is madeand the base metal of which sheet 19 is made begins to dissolve in themetal of which overlay strip 18 is made.

Fuse links made of the semi-finished link material shown in FIG. 6 anddescribed in conneoti ontwitl'i FIG. 6 have a mode of operation which isto that of the fuse links disclosed in the patent application ofFrederick J. Kozacka, Ser. No. 764,293; filed September 30, 1958 forTime-Lag Fuses, assigned to the assignee as the present application. Theparticular position of the strip 18 shown in FIG. 6 makes it possible toachieve a better or faster cooling action of the points of minimummethod shown in FIGS. 3 and 4 of aflixing the overlay strip 18 to thebase sheet 19 is also conducive to a better control of heating and tofaster rates of production than the method of providing an overlay on abase sheet disclosed in the above Kozacka application. Fuse linksmanufactured according to the present invention may be embodied toadvantage in the fuse structure disclosed in the above Kozack-aapplication.

Referring now to FIG. 7, showing a longitudinal sectlonal view of a fusecomprising a fuse link embodying this invention, reference numeral 21;hasigieen applied to indicate a pair of blade contacts to h the axiallyouter ends of a. ribbon type fuse link have been conductively connectedby such means as brazing. The above mentioned fuse link comprises basemetal 19 and overlay 18. Blade contacts 21 project into a tubular casing22 of insulating material closed on both ends by asbestos washers 23 andbrass caps 24. Each blade contact 21 is provided with a bore 26 intowhich a circularly bent spring 24 is inserted. The axially outer ends ofspring 24 project into holes in casing 22 arranged in alignment with theholes in blade contacts 21 which receive the center portions of springs24. Fuse link 18, 19 is submersed in a pulverulent arc-quenchinggier 25inside casing 22 as, for instance, quartz sand. link 18, 19 is providedwith three parallel lines 19a of circular perforations of which theaxially inner line of perforations is associated with overlay 18 of alow fusing point metal, e.g. tin. The base sheet 19 of link 18, 19 mayconsist of copper.

Strip 18 must be arranged parallel and close to one of the lines 19a ofperforations, preferably close to the center line of perforations. Itsposition cg be shifted from that shown FIG. 7 in such a W at one of theedges of strip 18 forms substantially a tangent to the circularperforations of the adjacent line 19a of perforations, thus doing awaywith the overlap between the strip 18 and the circular perforations.FIG. 7a shows the shifted position of strip 18. The aforementioned shiftof strip 18 has a marked effect upon the current rating of the fuse. Theaforementioned shift of strip 18 also eliminates the these figures itmust base and the tin overlay. This is no indication of structure buthas its optical reasons. In polishing the tin polishes down deeper thanthe copper. 'lg resulting geometric step appears black in all the photocrographs. In all photomicrographs the tin phase is lightly etched with2.5% nitric acid 5% acetic acid (glacial) in water.

The link whose cr0ss-section 18 shown in FIGS. 8 and 9 has beenmanufactured by placing a piece of tin on a copper strip, and thenplacing the copper strip on a hot plate until the tin is melted. FIGS. 8and 9 clearly show a copper tin intermetallic compound, probably etatin, at the interface between the two metals. The intermetallic compoundis present in the tin grain boundaries over distances of hit to /3 ofthe thickness of the tin overlay. The top surface of the tin, visibleatghe upper left of FIG. 8, allows a good estimation of the relativeextent of grain boundary penetration of the intermetallic compound.

The microstructure of FIGS. 10 and 11 shows good bonding between thecopper and tin surfaces virtually Without formation of an intermetalliccompound.

As mentioned above, the magnitude of the bonding current flowing throughthe low fusing point overlay material during the manufacturing processof the link material is of relatively minor significance in regard tothe present process. This applies to the bonding current flowing throughpellet 11 (FIGS. 1 and 2) as well as to the bonding current flowingthrough strip 18 (FIGS. 3 and 4). Several experiments were made todetermine the part played by that current in the present process. In oneof these experiments a composite top roll was substituted for theuniform top roll 14 shown in FIG. 4. The composite roll comprised anaxially inner portion of insulating material and two axially outerportions substantially identical with the axially outer portions orflange portions 14b shown in FIGS. 3 and 4 and described in connectiontherewith. That test set-up was found to work satisfactorily as long asthe axially inner insulating portion which precluded the flow of currentthrough strip 18 was adapted to firmly press strip 18 against base sheet19. The relatively small current flowing through the overlay metal maybe helpful where it is desired to change the shape of the overlay duringthe bonding process as, for instance, shown in FIGS. 1 and 2 anddescribed in connection therewith.

The term soldering is generally understood to mean joining two metalsurfaces by means of another metal, or alloy, that is applied in moltencondition. It will be apparent that the term soldering does not apply tothe present process if the above definition of soldering is adopted.

The process which has been described above is, in effect, a resistancewelding process applied to join a metal in the nature of soft low fusingpoint solder to a high fusing point metal such as copper or silveri Inorder to obtain good adherence of the overlay to the base metal thelatter must be as clean as possible. It is necessary or desirable topre-treat the high fusing point base metal with an appropriate flux asgenerally used in soldering operations. Since rosin flux may impair theconductivity of the set-up, acid flux should be used for carrying thisinvention into effect.

The strip-process illustrated in FIGS. 3 and 4 and described inconnection therewith calls for an overlay material which can readily bebent Without having a tendency to break. Tin with small additions ofantimony complied well with this particular requirement. The addition ofantimony may be in the order of 5% by weight.

It will be apparent from the foregoing that the method according to thisinvention minimizes alloying during the bonding operation of theoverlay, and makes it possible to achieve a complete control andstandardization of whatever minimal alloying occurs.

It will be understood that I have illustrated and described preferredembodiments of my invention and that various alterations may be made inthe details thereof without departing from the invention as defined inthe appended claims.

I claim:

1. A method for manufacturing fuse links for timelag fuses comprisingthe steps of superimposing under pressure upon a relatively thin sheetdefining zones of reduced cross-sectional area and having a relativelylarge surface area and being made of a metal having a relatively highfusing point and a relatively high conductivity immediately adjacent oneof said zones of reduced crosssectional area a relatively thick overlayhaving a relatively small surface area and being made of a low fusingpoint metal in the nature of soft solder; and of passing an electriccurrent through said sheet adjacent the interface between said sheet andsaid overlay, said current being sufficiently high and of sufficientlylong duration to cause fusion of said overlay at said interface, andsaid current being sufficiently small and of sufficiently short durationto preclude fusion of said overlay at portions thereof remote from saidinterface.

2. A method for manufacturing fuse links for timelag fuses comprisingthe steps of superimposing under pressure upon a relatively thin sheetdefining zones of reduced cross-sectional area and having a relativelylarge surface area and being made of a metal having a relatively highfusing point and a relatively high conductivity immediately adjacent oneof said zones of reduced cross-sectional area, a relatively thickoverlay having a relatively small surface area and being made of a lowfusing point metal in the nature of soft solder; of passing an electriccurrent through said sheet adjacent the interface between said sheet andsaid overlay and of causing said current to be denser at the surface ofsaid sheet supporting said overlay than on the surface thereof remotefrom said overlay, said current being sufiiciently high and ofsufficiently long duration to cause fusion of said overlay at saidinterface and said current being sufficiently low and of sufficientlyshort duration to preclude fusion of said overlay at portions thereofremote from said interface.

3. A method for manufacturing fuse links for timelag fuses comprisingthe steps of causing engagement under pressure between a relatively thinsheet of a metal having a relatively high fusing point and a relativelyhigh conductivity and a pre-measured pellet of a metal having arelatively low fusing point and a relatively low conductivity, and ofpassing an electric current through said sheet adjacent the interfaceformed between said sheet and said pellet, said current beingsufficiently high and of sufficiently long duration to cause fusion ofsaid pellet at said interface, and said current being sufficiently smalland of sufficiently short duration to preclude fusion of said pellet atportions thereof remote from said interface.

4. A method for manufacturing fuse links for timelag fuses comprisingthe steps of causing engagement under pressure between a pro-measuredrelatively thick pellet of a low fusing point metal in the nature ofsoft solder and a relatively thin sheet of copper, and of establishing aflow of current through said sheet around said pellet whileprogressively increasing the pressure between said pellet and saidsheet, said current being sufliciently high and of sufficiently longduration to cause fusion of said pellet at the interface formed betweensaid pellet and said sheet, and said current being sufficiently smalland of sufficient short duration to preclude fusion of said pellet atportions thereof remote from said interface.

5. A method for manufacturing fuse links for timelag fuses comprisingthe steps of causing engagement under pressure between a pre-measuredrelatively thick pellet of a low fusing point metal in the nature ofsoft solder and a relatively thin sheet of a metal having a relativelyhigh fusing point and a relatively high conductivity, of establishing aclosed electric circuit including said pellet and said sheet and passingan electric current through said circuit while progressively increasingthe pressure between said pellet and said sheet, and of establishing insaid circuit a current path through said sheet immediately adjacent theinterface formed between said pellet and said sheet, said current pathshunting the path of the current through said pellet and having asmaller electric resistance than said pellet.

6. A method for manufacturing fuse links for timelag fuses comprisingthe steps of placing a relatively thick pro-measured pellet of a metalin the nature of soft solder into a cavity defined by a first electrode;of causing engagement between a portion of said pellet protrudingoutside said cavity and a relatively thin sheet of a metal having arelatively high fusing point and a relatively high conductivity and ofcausing engagement between said sheet and a second electrode; ofapplying sufficient pressure by said first electrode and said secondelectrode upon said pellet and said sheet to compress said pellet bysaid sheet into said cavity in said first electrode; and ofsimultaneously passing an electric current through said first electrode,said sheet and said second electrode, said current being sufficientlyhigh and of sufficient duration to cause fusion of said pellet at theinterface formed between said pellet and said sheet.

7. A method for manufacturing fuse links for timelag fuses comprisingthe steps of causing engagement under pressure between a relatively thinsheet of a metal having a relatively high fusing point and a relativelyhigh conductivity and a relatively thick strip of a low fusing pointmetal in the nature of soft solder; of establishing an electric circuitthrough said sheet comprising a pair of parallel current paths in saidsheet each situated adjacent the interface formed between said sheet andsaid strip; and of passing an electric current through said parallelcurrent paths, said current being sufficiently high and of sufilcientlylong duration to cause said strip to adhere to said sheet, and saidcurrent being sufficiently small and of sufficiently short duration topreclude total fusion of said strip.

8. A method for manufacturing fuse links for timelag fuses comprisingthe steps of superimposing under pressure upon a metal in sheet-formhaving a relatively high fusing point and a relatively high conductivitya pie-measured amount of a metal in nature of soft solder; ofmaintaining pressure between said first mentioned metal and said secondmentioned metal and simultaneously establishing a local heatconcentration at the interface between said first mentioned metal andsaid second mentioned metal by passing an electric current through saidfirst mentioned metal immediately adjacent said interface, said currentbeing sufficiently high and of sufiiciently long duration to causefusion of said first mentioned metal at said interface and formation ofa fusion joint with said second mentioned metal at said interface, andsaid current being sufficiently small and of sufficiently short durationto preclude fusion of said first mentioned metal at points remote fromsaid interface.

9. A method for manufacturing fuse links for timelag fuses comprisingthe steps of placing a pre-measured amount of a metal in the nature ofsoft solder and a sheet of a metal having a relatively high fusing pointand a relatively high conductivity between a pair of spaced electrodesforming part of the secondary circuit of a transformer; of reducing thespacing between said pair of electrodes and applying pressure with saidpair of electrodes upon said first mentioned metal and said sheet; andof causing the flow of an electric current in said secondary circuittransversely across said sheet on both sides of the interface formedbetween said first mentioned metal and said sheet to heat said interfacefrom two juxtaposed regions, said current being sufficiently high and ofsufficiently long duration to cause fusion of said first mentioned metalat said interface, and said cur rent being sufficiently small and ofsufficiently short duration to preclude fusion of said first mentionedmetal at points thereof remote from said interface.

10. A method for manufacturing fuse links for timelag fuses comprisingthe steps of superimposing under pressure upon a relatively wide,relatively thin sheet of a metal having a relatively high fusing pointand a relatively high conductivity and defining a substantially straightzone of reduced cross-sectional area, parallel to said zone of reducedcross-sectional area by a rolling motion successively contiguous pointsof a relatively narrow, relatively thick strip of a metal in the natureof soft, low melting point solder; of successively passing an electriccurrent through aligned points of said sheet adjacent the interfaceformed between said sheet and said strip, said current beingsufficiently high to cause said strip to fuse at the interface formedbetween said strip and said sheet and to be bonded to said sheet, andsaid current being sufficiently low to preclude fusion of said stripthroughout the entire mass thereof.

11. A method for manufacturing fuse links for timelag fuses comprisingthe steps of superimposing under pressure upon a relatively thin andrelatively wide sheet of a metal having a relatively high fusing pointand a relatively high conductivity and defining a straight line ofreduced cross-sectional area, successively parallel to said straightline a relatively thick and relatively narrow strip of a metal in thenature of soft low melting point solder; of passing an electricalternating current through successively engaging points of said stripand said sheet while maintaining pressure at said points; said currenthaving a sufliciently high R.M.S. value to cause said strip to fuse andto be bonded at the interface between said strip and said sheet to saidsheet, and said current having a sufliciently low R.M.S. value topreclude fusion of portions of said strip remote from said interface.

12. A method for manufacturing fuse links for timelag fuses comprisingthe steps of superimposing under pressure a relatively thin andrelatively wide sheet of copper defining a straight line of reducedcross-sectional area, successively parallel to said straight line arelatively thick and relatively narrow strip of a metal in the nature ofsoft, low fusing point solder; and of causing sequential local heatingof aligned points of said sheet when being sequentially engaged by saidstrip by passing an electric current through said sheet adjacent theinterface between said sheet and said strip, said current beingsufliciently high to result in formation of a bond between said strip tosaid sheet, and said current being sufiiciently low to preclude fusionof said strip at points remote from said interface.

13. A method for manufacturing fuse links as specifled in claim 12wherein said strip consists of tin with a small addition of antimony.

References Cited in the file of this patent UNITED STATES PATENTS444,928 Thomson Jan. 20, 1891 1,278,234 Sessions Sept. 10, 19181,541,513 Knoop June 9, 1925 1,806,188 Adams May 19, 1930 2,306,772Benson Dec. 29, 1942 2,691,208 Brennan Oct. 12, 1954 2,793,423 StumbockMay 28, 1957 2,824,359 Rhodes Feb. 25, 1958 2,879,587 Mushovic Mar. 31,1959

1. A METHOD FOR MANUFACTURING FUSE LINKS FOR TIMELAG FUSES COMPRISINGTHE STEPS OF SUPERIMPOSING UNDER PRESSURE UPON A RELATIVELY THIN SHEETDEFINING ZONES OF REDUCED CROSS-SECTIONAL AREA AND HAVING A RELATIVELYLARGE SURFACE AREA AND BEING MADE OF A METAL HAVING A RELATIVELY HIGHFUSING POINT AND A RELATIVELY HIGH CONDUCTIVITY IMMEDIATELY ADJACENT ONEOF SAID ZONES OF REDUCED CROSS-SECTIONAL AREA A RELATIVELY THICK OVERLAYHAVING A RELATIVELY SMALL SURFACE AREA AND BEING MADE OF A LOW FUSINGPOINT METAL IN THE NATURE OF SOFT SOLDER; AND OF PASSING AN ELECTRICCURRENT THROUGH SAID SHEET ADJACENT THE INTERFACE BETWEEN SAID SHEET ANDSAID OVERLAY, SAID CURRENT BEING SUFFICIENTLY HIGH AND OF SUFFICIENTLYLONG DURATION TO CAUSE FUSION OF SAID OVERLAY AT SAID INTERFACE, ANDSAID CURRENT BEING SUFFICIENTLY SMALL AND OF SUFFICIENTLY SHORT DURATIONTO PRECLUDE FUSION OF SAID OVERLAY AT PORTIONS THEREOF REMOTE FROM SAIDINTERFACE.